The invention relates to a common-mode signal sensor for supplying a common-mode signal in response to a difference signal, comprising: a first input terminal and a second input terminal for receiving the difference signal, an output terminal for supplying the common-mode signal, a first and a second transistor, each having a first main electrode, a second main electrode and a control electrode, the first main electrodes of the first and the second transistor being interconnected in a first node and the control electrode of the first transistor being coupled to the first input terminal, a first current source coupled to the first node to supply a first bias current, a third and a fourth transistor, each having a first main electrode, a second main electrode and a control electrode, the first main electrodes of the third and the fourth transistor being interconnected in a second node and the control electrode of the fourth transistor being coupled to the second input terminal, and a second current source coupled to the second node to supply a second bias current.
Such a common-mode signal sensor is known from the article "CMOS Voltage to Current Transducers", IEEE Transactions on Circuits and Systems, Vol. CAS-32, No. 11, November 1985, pp. 1097-1104, FIG. 6. Common-mode signal sensors are used in common-mode rejection circuits for operational amplifiers, transconductors and similar electronic circuits. A known technique for measuring the common-mode signal content of a difference signal utilises the voltage on the node common to the first main electrodes of a standard differential pair whose control electrodes are arranged to receive the difference signal. This arrangement has the drawback that said voltage contains not only the common-mode signal but also the threshold voltage of the transistors which are used and, in addition, a part of the difference signal. The common-mode signal sensor known from said article employs two differential pairs. The first and the second transistor with the first current source constitute the first differential pair. The third and the fourth transistor with the second current source constitute the second differential pair. The control electrodes of the first and the fourth transistor are connected to the first and the second input terminal and receive the input signal whose common-mode voltage is to be measured. In a third node the control electrodes and the second main electrodes of the second and the third transistor are all connected to a third current source supplying a current equal to that of the first and the second current source. The voltage on the third node is a measure of the common-mode voltage of the input signal and no longer contains the threshold voltage of the transistors. However, this common-mode signal sensor has the drawback of a limited difference signal range because the voltages on the first and the second node do not increase symmetrically with an increasing difference voltage on the input terminals.